Altimeter Settings Explained: QNH, QFE and Standard 1013 hPa

Before every take-off, ATC gives the pilot a number like "QNH 1008". Climbing out of Delhi, the crew later switch the altimeter to 1013 and start talking in "flight levels". On a short hop from Kolkata the switch happens lower. Why do pilots keep changing one small knob, and what happens if they forget? This guide explains altimeter settings in plain language: QNH, QFE, the standard setting of 1013.25 hPa, transition altitude and the classic errors, using FAA, ICAO and Airports Authority of India sources.

This is neutral, educational information, not flight training. For Indian student pilots, altimetry (ISA, QNH, QFE, 1013.25 hPa, pressure altitude and flight levels) is part of the DGCA CPL Aviation Meteorology syllabus in CAR Section 7, Series B, Part IV. Check the current version on dgca.gov.in.

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What an altimeter really measures

A normal aircraft altimeter does not measure height with radar or GPS. The FAA's Pilot's Handbook of Aeronautical Knowledge, Chapter 8 describes it as an aneroid barometer: a stack of sealed metal wafers that expand as outside air pressure falls and contract as it rises. Gears turn that movement into an altitude reading in feet.

Because air pressure falls as you climb, lower pressure is shown as a higher altitude. The catch is that sea-level pressure changes from day to day and place to place. The FAA notes that the reading is only correct when conditions match the standard atmosphere, so the pilot must dial the right pressure into a small scale on the instrument (in the US often called the Kollsman window). That number is the altimeter setting.

The standard atmosphere and 1013.25 hPa

To give every altimeter a common reference, ICAO defines the International Standard Atmosphere (ISA). At mean sea level, ISA pressure is 1013.25 hectopascals (hPa), which equals 29.92 inches of mercury, and the temperature is 15°C. India's AIP (GEN 2.1) lists altimeter settings in hectopascals, while the US uses inches of mercury. Air traffic control and charts usually round the standard setting to 1013 or 1013.2 hPa.

A useful rule of thumb from the FAA: near sea level, 1 inch of mercury is roughly 1,000 feet of altitude. Since 1 inch is about 33.9 hPa, that works out to roughly 27 to 30 feet per hectopascal. So a 10 hPa mistake on the altimeter means an error of roughly 300 feet.

QNH, QFE and standard: the three settings

The "Q" names come from the old radio Q-code, and they are still used worldwide.

• QNH: the local pressure reduced to mean sea level. With QNH set, the altimeter shows your altitude above mean sea level, and on the ground it shows the aerodrome's elevation. This is the setting used for take-off, landing and flying near airports, and the one terrain and obstacle heights on charts are measured against.
• QFE: the actual pressure at the aerodrome. With QFE set, the altimeter reads zero at the aerodrome elevation (or runway threshold) and shows height above that point. It is used in some training circuits and some military flying. India's AIP centres on QNH.
• Standard (1013.25 hPa / 29.92 inHg): everyone sets the same value, so the altimeter shows pressure altitude. Pilots often call this "standard" or "STD" (some use the Q-code QNE). The FAA defines pressure altitude as the reading with the scale set to 29.92 inHg.

A simple way to remember it: QNH gives "height above the sea", QFE gives "height above the field", and standard gives a level that is the same for every aircraft in the sky.

Flight levels: why high-flying aircraft all use 1013

An airliner crossing India passes many weather stations, each with a different QNH. If every pilot kept changing to the nearest local QNH, two aircraft close to each other might be using different settings, and their altimeters would disagree. Separation depends on everyone measuring altitude the same way.

So above a certain point, everyone sets the standard pressure and flies flight levels. India's AIP (ENR 1.7, Altimeter Setting Procedures) states that flight level 0 is at the 1013.2 hPa pressure level, and that consecutive flight levels are separated by a pressure interval equal to 500 feet in the standard atmosphere. FL 100 is shown as 10,000 feet on a standard-set altimeter, and FL 350 as 35,000 feet.

Transition altitude, transition level and the transition layer

• Transition altitude: the altitude at or below which aircraft fly on QNH and report altitude.
• Transition level: the lowest flight level available above the transition altitude. At or above it, aircraft fly on standard pressure and report flight levels.
• Transition layer: the airspace between the two.

According to India's AIP ENR 1.7, a transition altitude is published for each aerodrome and none is less than 4,000 feet. The values in the AIP table vary with terrain. In the edition we checked, Kolkata is 4,000 feet, Delhi (IGI) is 5,000 feet, Mumbai (CSI) is 6,000 feet, and Leh is 24,000 feet. Pilots always check the current AIP and approach charts rather than relying on a blog.

The AIP also explains the routine. QNH is given in the taxi clearance. While climbing, aircraft report altitude until the transition altitude, then switch to standard and flight levels. On descent, they use flight levels until the transition level and then switch to QNH. QNH is also given with approach and landing instructions, and pilots read it back to ATC.

Other countries do this differently. In the United States, for example, the transition altitude is a uniform 18,000 feet, so Indian students training abroad need to learn the local rules.

The two classic errors: "high to low, look out below"

1. Pressure error

If an aircraft flies from an area of high pressure to an area of low pressure without resetting the altimeter, the reading stays the same while the aircraft actually gets lower. The FAA's memory aid is "Going from a high to a low, look out below." The reverse (low to high) puts the aircraft higher than indicated.

A worked example using the 30 ft per hPa rule: a pilot sets QNH 1013 and flies towards an area where the QNH is 1003, without updating. The altimeter over-reads by about 10 × 30 = 300 feet, so the aircraft is about 300 feet lower than the instrument shows. Near hills, that matters.

2. Temperature error

Setting the right pressure does not correct for temperature. The FAA explains that cold air is denser, so in colder-than-standard air the aircraft's true altitude is lower than indicated, and in warmer-than-standard air it is higher. Its second memory aid is "From hot to cold, look out below." In very cold conditions, especially over mountains, pilots apply cold-temperature corrections.

Types of altitude you will meet in DGCA papers

• Indicated altitude: what the altimeter shows with the current setting.
• True altitude: actual height above mean sea level.
• Absolute altitude: height above the ground directly below (AGL).
• Pressure altitude: the reading with 1013.25 hPa / 29.92 inHg set.
• Density altitude: pressure altitude corrected for non-standard temperature. It drives aircraft performance: on a hot day density altitude is higher, so take-off runs are longer.

These definitions follow the FAA handbook and match the altimetry topics in the DGCA Meteorology syllabus. Our guide on how to pass DGCA CPL Meteorology shows where altimetry fits, and how to read a METAR shows where the QNH in a weather report comes from (the "Q1008" group).

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Frequently asked questions

What is QNH in aviation?

QNH is the local air pressure reduced to mean sea level. With it set, the altimeter shows altitude above mean sea level and reads the airport's elevation on the ground.

What is the difference between QNH and QFE?

QNH makes the altimeter show height above mean sea level. QFE makes it read zero at the aerodrome elevation, so it shows height above the aerodrome.

Why is 1013.25 hPa called standard pressure?

It is the mean sea-level pressure in ICAO's International Standard Atmosphere, equal to 29.92 inches of mercury. Setting it gives pressure altitude, which is used for flight levels.

What is the transition altitude in India?

It is set for each aerodrome in India's AIP (ENR 1.7), and none is below 4,000 feet. It is higher near high terrain, for example 24,000 feet at Leh in the edition we checked. Always use the current AIP and charts.

What does "high to low, look out below" mean?

Flying from high to low pressure without resetting the altimeter makes it over-read, so the aircraft is lower than it shows. About 30 feet per hPa is a handy estimate near sea level.

Is altimetry asked in DGCA exams?

Altimetry, including ISA, QNH, QFE, 1013.25 hPa, pressure altitude and flight levels, is listed in the DGCA CPL Aviation Meteorology syllabus (CAR Section 7, Series B, Part IV). Exact questions vary, so study from the official syllabus.

Wrap-up

An altimeter is a pressure gauge, so its reading is only as good as the setting dialled into it. QNH gives altitude above sea level for take-off, landing and terrain clearance. QFE gives height above the runway. Standard pressure, 1013.25 hPa, gives every high-flying aircraft the same reference so flight levels keep them apart. Learn when to switch between them, and remember "high to low, look out below", and one of the trickiest Meteorology topics becomes simple.

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Related: How to read a METAR · How to pass DGCA CPL Meteorology · How VOR navigation works · How heavy aircraft fly · DGCA exams explained

General educational information only, not flight training. Altimeter explanations follow the FAA Pilot's Handbook of Aeronautical Knowledge (Chapter 8) and the ICAO Standard Atmosphere; Indian procedures and transition altitudes are from the Airports Authority of India eAIP (ENR 1.7 and GEN 2.1), checked on 19 October 2026. Always use the current AIP, charts and DGCA syllabus (dgca.gov.in).